1. Field of the Invention
The present invention relates generally to radio-frequency identification (RFID) technology, and in particular relates to antenna systems for passive RFID tags.
2. Technical Background
Radio-frequency identification (RFID) is a remote recognition technique that utilizes RFID tags having information stored therein, usually in an integrated circuit (IC). The stored information is retrievable via RF communication between the RFID tag and a RFID reader. A RFID reader is the device that communicates with one or more RFID tags, which may be placed on or attached to different objects. RFID systems may utilize a hand-held RFID reader that when brought sufficiently close to a RFID tag is able to read a RFID tag signal either emitted by or backscattered from the tag. RFID systems are used for a variety of applications, including inventory management and product tracking in a number of different industries, as well as in libraries and hospitals.
RFID tags generally come in three varieties: passive, semi-passive, and active. Passive RFID tags have no energy or power source of their own and operate by harvesting energy from the RFID reader's RF field. Passive tags communicate with the RFID reader by modulating and back-scattering RF radiation emitted by the RFID reader.
A passive RFID tag essentially comprises an antenna connected to an integrated circuit (IC). The antenna is designed to operate at the RFID reader operating frequency f (wavelength λ) and serves the dual purpose of capturing power from the reader RF field to power up and operate the IC, and to reflect some of the incident RF field back to the RFID reader in a modulated fashion to communicate information, such as its identification number.
The antenna may have to satisfy a number of design and system constraints on its size and configuration, while still providing a required level of performance to fulfill its function. Antennas currently used for passive RFID tags typically have one or more unsuitable characteristics, such as being too large, the antenna arms extend away from each other, and unacceptable performance sensitivity when several RFID tags are in close proximity with each other, i.e., separated by much less than the RF wavelength λ used by the RFID reader.